Innovative Portable Camera can See Polarization

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In 1935, the first full-length movie produced with the advanced, three-color process of Technicolor premiered, and the New York Times stated that “it produces in the spectator all the excitement of standing upon a peak … and glimpsing a strange, beautiful, and unexpected new world.”

Structures in the human eye can distinguish color and light. However, humans are not equipped to see polarization, the direction in which light vibrates. But polarization can offer a significant amount of information regarding the objects with which it interacts. Cameras that can image polarized light reflected off surfaces are presently used to identify material stress, improve contrast for object detection, and examine surface quality for scratches or dents.

However, similar to the early color cameras, present-generation polarization-sensitive cameras are bulky. Furthermore, they mostly depend on moving parts and they are costly, severely restricting their potential applications.

Recently, scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have designed a very compact, portable camera that can image polarization in one shot. The tiny camera could be employed in the vision systems of autonomous vehicles, onboard planes, or satellites to explore atmospheric chemistry, or be used to detect concealed objects. The study has been published in Science.

“This research is game-changing for imaging,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, the paper’s senior author. “Most cameras can typically only detect the intensity and color of light but can’t see polarization. This camera is a new eye on reality, allowing us to reveal how light is reflected and transmitted by the world around us.”

Polarization is a feature of light that is changed upon reflection off a surface. Based on that change, polarization can help us in the 3D reconstruction of an object, to estimate its depth, texture, and shape, and to distinguish man-made objects from natural ones, even if they’re the same shape and color.

Paul Chevalier, Study Co-Author and Postdoctoral Fellow, SEAS, Harvard University

To unravel that powerful domain of polarization, Capasso and his team exploited the potential of metasurfaces, nanoscale structures that act together with light at wavelength-size scales.

“If we want to measure the light’s full polarization state, we need to take several pictures along different polarization directions,” said Noah Rubin, first author of the paper and a graduate student in the Capasso Lab.

He added, “Previous devices either used moving parts or sent light along multiple paths to acquire the multiple images, resulting in bulky optics. A newer strategy uses specially patterned camera pixels, but this approach does not measure the full polarization state and requires a non-standard imaging sensor. In this work, we were able to take all of the optics needed and integrate them in a single, simple device with a metasurface.”

With a new insight of how polarized light relates with objects, the scientists developed a metasurface that uses a range of subwavelength-spaced nanopillars — miniature structures that can be assembled together in lattices — to direct light according to its polarization. The light then forms four images, each one revealing a different facet of the polarization. Taken together, these deliver a complete snapshot of polarization at every pixel.

The entire device measures about 2 cm in length and is no more complex than a camera on a smartphone. Along with an attached lens and protective case, the device takes the space of a small lunch box. The scientists tested the camera by exposing the defects in injection-molded plastic objects, taking it outside to film the polarization off car windshields, and even clicking selfies to show how a polarization camera can envisage the 3D contours of a face.

This technology could be integrated into existing imaging systems, such as the one in your cellphone or car, enabling the widespread adoption of polarization imaging and new applications previously unforeseen.

Noah Rubin, Study First Author and Graduate Student, Capasso Lab, SEAS, Harvard University

“This research opens an exciting new direction for camera technology with unprecedented compactness, allowing us to envision applications in atmospheric science, remote sensing, facial recognition, machine vision, and more,” said Capasso.

The Harvard Office of Technology Development has protected the intellectual property associated with this research and is looking for commercialization opportunities.

The research was co-authored by Gabriele D’Aversa, Zhujun Shi and Wei Ting Chen. It was aided by the National Science Foundation, the Air Force Office of Scientific Research, a Physical Sciences & Engineering Accelerator grant from Harvard University’s Office of Technology Development, Google Accelerated Science, and King Abdullah University of Science and Technology. This research was conducted partly at Harvard’s Center for Nanoscale Systems.

Researchers at SEAS have developed a highly compact, portable camera that can image polarization in a single shot. (Video courtesy of SEAS)

Source: https://www.harvard.edu/

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